The invention relates to a process for catalytic dehydration of propane to form propylene in accordance with the following formula:C3H8C3H6+H2  (1)from various sources of propane, which may also contain other gas components. The chemical reaction (1), which normally takes place in a gaseous phase at a temperature ranging from 540° C. to 820° C., is a strong endothermic equilibrium reaction the conversion rates of which are subject to thermodynamic limits and which depend on the specific partial pressures and the temperature. The dehydration reaction is promoted by low partial pressures of the hydrocarbons and by high temperatures. Cracked products form in side reactions and cause deposits of hydrocarbons on the catalyst so that the latter is deactivated and thus requires a cyclic regeneration during plant operation.
If dehydration takes place in a catalyst bed operated in an adiabatic manner, the endothermic reaction causes a gradual temperature drop over the whole length of the catalyst bed. Hence, the conversion rate in the catalyst bed is restricted to a certain degree so that several catalyst beds are required to obtain the high conversion rates desired and re-heating must be provided downstream of each catalyst bed.
The catalytic dehydration of paraffin to obtain olefin may in fact take place in heated or isothermal catalyst beds, too. U.S. Pat. No. 5,235,121, for example, describes a process in which the input mixture consists of light paraffins and water vapour and is fed to a tubular reactor with external firing, i.e. the catalyst bed is a heated fixed bed. The catalyst used in that process is of such a nature that the presence of water vapour cannot initiate a steam reforming process, i.e. there is no reaction of hydrocarbons with water vapour producing CO, CO2 and H2. The catalyst undergoes cyclic regeneration. DE 198 58 747 A1 describes a similar process.
Heating the catalyst bed or implementing an isothermal operating mode permit high conversion rates in a bulk catalyst bed. The disadvantage of this method, however, is that such very high conversion rates can only be achieved at high temperatures on account of the position of thermodynamic equilibrium, which also entails a reduction of the selectivity.
The operating mode in the presence of water vapour as described above has the advantage that it reduces the partial pressure of hydrocarbons and consequently increases the conversion rate. Moreover, the use of steam helps to convert part of the hydrocarbon deposits on the catalyst to form CO2 and to obtain prolonged intervals between the regeneration cycles. However, the addition of too much steam is disadvantageous since it causes a substantial increase in volume of the gas stream, which requires a larger amount of investment and deteriorates the process efficiency. Furthermore the danger of steam reforming of hydrocarbons increases, which entails product losses and/or decrease in yield. The amount of steam that can be added without encountering the said problems entirely depends on the absolute pressure of the reaction and on the dehydration catalyst used.
A further possibility to overcome the thermodynamic limitation of the conversion rate under equilibrium conditions is to selectively burn part of the hydrogen obtained by dehydration, i.e. by the addition of oxygen—termed SHC for “Selective Hydrogen Combustion”—and thereby shifting the dehydration equilibrium to a higher conversion rate. EP 0 799 169 B1, for example, describes a reactor for such a dehydration process combined with SHC, in which a paraffin/oxygen mixture is sent via a first catalyst suitable for dehydration as well as selective oxidation of hydrogen obtained, further oxygen being introduced into an intermediate chamber of the reactor, and a second catalyst likewise being provided for dehydration and selective oxidation of hydrogen obtained. The process in accordance with EP 0 799 169 B1 takes place in an autothermal mode, the exothermic reaction of hydrogen with oxygen supplying the energy required to carry out the endothermic dehydration reaction (1).
Furthermore, document WO 96/33150, for example, describes a process in which the paraffin mixture is first dehydrated in a first step, oxygen being subsequently added and, at least, a second step is provided for making this oxygen react with the hydrogen released by dehydration in order to form water vapour. At least one part stream of the product obtained undergoes post-dehydration so that non-reacted paraffins can be dehydrated. It is also suggested that a return stream to the first step be installed.
These two processes have the disadvantage that the addition of oxygen and the exothermic selective oxidation of hydrogen cause very high temperatures, a fact that entails a reduction of the selectivity of catalytic dehydration.
The problem of overheating the hydrogen oxidation and with it the downstream dehydration can be solved by arranging an intercooler upstream of the selective hydrogen oxidation to reduce the inlet temperature of the second catalyst bed. Document U.S. Pat. No. 4,599,471, for example, suggests such an inter-cooling which may be of the indirect or direct type. Direct cooling can be carried out with the aid of inert gases, such as nitrogen, helium etc. or steam.
Temperature adjustment, however, by indirect cooling is a demerit because it requires a firm heat exchanger installation, which does not permit a specific temperature control of the catalyst bed regeneration, or it is necessary to install a device for temporary uncoupling of the heat exchangers, for example, by means of a by-pass that can be shut off by a valve. The latter solution would be a rather expensive configuration in view of the large pipe cross-sections and the high operating temperature of about 500° C. to 650° C. due to the dehydration. Direct cooling by inert gas is a real demerit because such gases would later have to be separated from the product in costly process steps during product makeup. Direct cooling using steam is a disadvantage as steam is not inert in the reaction (as described above) and because the cooling measure entails a certain steam-to-hydrocarbon ratio depending on the type of cooling. An intense cooling thus substantially increases the amount of steam, which is detrimental to the process.
Document WO 2004/039920 also describes a process in which water as well as water vapour are added for the reason indicated above. All of the processes named here are not suitable for the use of propane sources that are contaminated by oxygen or other gas components without performing a previous treatment of the propane stream from such a source, because otherwise heating the input stream which contains propane and oxygen to the reactor inlet temperature would already trigger off a non-catalytic and thus non-selective reaction of the oxygen with propane, thus causing loss of product yield.
A further demerit of all processes described here consists in the fact that hydrogen is present in the product stream of the process. For a later exploitation of the olefin product, the hydrogen must be separated in a relatively sophisticated and expensive gas separation step. This is a specific disadvantage when the olefin product is exploitable at a comparatively low degree of purity, i.e. if the gas treatment process can be performed in a very simple manner to satisfy any other requirement.